1. Technical Field
The invention relates generally to reduced pressure fining, a process for removing trapped bubbles in molten glass. More specifically, the invention relates to a tubing system for conveying molten glass through a vacuum chamber while maintaining an airtight condition within the vacuum chamber.
2. Background Art
In industrial glassmaking, a glass batch is made by mixing in blenders a variety of raw materials obtained from properly sized, cleaned, and treated materials that have been pre-analyzed for impurity. Recycled glass called cullet may also be mixed with the raw materials. For the most commonly produced soda-lime glass, these raw materials include silica (SiO.sub.2), soda (Na.sub.2 O), lime (CaO), and various other chemical compounds. The soda serves as a flux to lower the temperature at which the silica melts, and the lime acts as a stabilizer for the silica. A typical soda-lime glass is composed of about seventy percent silica, fifteen percent soda, and nine percent lime, with much smaller amounts of the various other chemical compounds. The glass batch is conveyed to a "doghouse", which is a hopper at the back of the melting chamber of a glass melting furnace. The glass batch may be lightly moistened to discourage segregation of the ingredients by vibrations of the conveyor system or may be pressed into pellets or briquettes to improve contact between the particles.
The glass batch is inserted into the melting chamber by mechanized shovels, screw conveyors, or blanket feeders. The heat required to melt the glass batch may be generated using natural gas, oil, or electricity. However, electric melting is by far the most energy efficient and clean method because it introduces the heat where needed and eliminates the problem of batch materials being carried away with the flue gases. To ensure that the composition of the molten glass is homogenous throughout, the molten glass is typically stirred together in a conditioning chamber that is equipped with mechanical mixers or nitrogen or air bubblers. The molten glass is then carried in a set of narrow channels, called forehearth, to the forming machines. In the melting chamber, large quantities of gas can be generated by the decomposition of the raw materials in the batch. These gases, together with trapped air, form bubbles in the molten glass. Large bubbles rise to the surface, but, especially as the glass becomes more viscous, small bubbles are trapped in the molten glass in such numbers that they threaten the quality of the final product. For products requiring high quality glass, e.g., liquid crystal displays, the trapped bubbles are removed from the molten glass prior to feeding the molten glass into the forming machines.
The process of removing bubbles from molten glass is called fining. One method for fining glass involves adding various materials known as fining agents to the glass batch prior to mixing in the blenders. The primary purpose of the fining agents is to release the gases in the molten glass when the molten glass is at the proper fining temperature. The released gases then diffuse into gas bubbles in the molten glass. As the bubbles become larger, their relative buoyancy increases, causing them to rise to the surface of the molten glass where they are released. The speed at which the bubbles move through the molten glass may be increased by reducing the viscosity of the molten glass, and the viscosity of the molten glass can be reduced by increasing the temperature of the molten glass. An effective fining agent for atmospheric pressure, glass melting and fining processes should be able to release a large amount of fining gases as the temperature of the molten glass is increased to the temperature range where the viscosity of the molten glass is sufficiently low, i.e., 1300.degree. C. to 1500.degree. C. for soda-lime glass. An example of a fining agent that is suitable for use with soda-lime glass is sodium sulfate (Na.sub.2 SO.sub.4).
Another method for fining glass involves passing the molten glass through a low pressure zone to cause the bubbles in the molten glass to expand and rise quickly to the surface of the glass. This process is typically referred to as reduced pressure fining or vacuum fining. There are various configurations of reduced pressure finers. U.S. Pat. No. 5,849,058 to Takeshita et al. discloses the general structure of a siphon-type reduced pressure finer. The reduced pressure finer, as shown in FIG. 1, includes a vacuum vessel 1 disposed in vacuum housing 2. The vacuum vessel 1 has one end connected to an uprising pipe 3 and another end connected to a downfalling pipe 4. The uprising pipe 3 and the downfalling pipe 4 are made of platinum, a material that can withstand the high temperature of the molten glass and that is not easily corroded. The vacuum vessel 1, the uprising pipe 3, and the downfalling pipe 4 are heated by electricity. An insulating material 5 is provided around the vacuum vessel 1, the uprising pipe 3, and the downfalling pipe 4. Typically, the insulating material 5 consists generally of insulating bricks and doubles as a structural support for the uprising pipe 3 and the downfalling pipe 4. The bottom ends of the uprising pipe 3 and the downfalling pipe 4 that are not connected to the vacuum vessel 1 extend through the vacuum housing 2 into the storage vessels 6 and 7, respectively. The storage vessel 6 is connected to receive molten glass from a glass melting furnace (not shown).
Flow of molten glass through the uprising pipe 3, the vacuum vessel 1, and the downfalling pipe 4 follows the siphon principle. Accordingly, the liquid surface of the molten glass in the vacuum vessel 1 is higher than the liquid surface of the molten glass in the storage vessel 6, and the pressure in the vacuum vessel 1 is lower than the pressure in the storage vessel 6. The pressure in the vacuum vessel 1 is related to the elevation of the liquid surface of the molten glass in the vacuum vessel 1 with respect to the liquid surface of the molten glass in the storage vessel 6. The height of the liquid surface of the molten glass in vacuum vessel 1 above the liquid surface of the molten glass in the storage vessel 6 is set based on the desired fining pressure and the rate at which molten glass is flowing into the vacuum vessel 1. The molten glass with the trapped bubbles is transferred from the glass melting furnace (not shown) into the storage vessel 6. Because the pressure in the vacuum vessel 1 is less than the pressure in the storage vessel 6, the molten glass in the storage vessel 6 rises through the uprising pipe 3 into the vacuum vessel 1. The pressure in the vacuum vessel 1 is brought to reduced pressure condition of less than the atmospheric pressure, typically 1/20 to 1/3 atmospheric pressure. As the molten glass passes through the vacuum vessel 1 and encounters the reduced pressure, the bubbles in the molten glass expand and quickly rise to the surface of the molten glass, creating a foam layer in the headspace 8. The refined glass descends into the storage vessel 7 through the downfalling pipe 4.
The vacuum housing 2 must be designed to minimize inspiration of air during the fining process. The locations where the uprising pipe 3 and the downfalling pipe 4 exit the vacuum housing 2 must be sealed to ensure an airtight condition within the vacuum housing 2. However, because the uprising pipe 3 and the downfalling pipe 4 expand as they are heated, it is difficult to maintain a reliable seal between the wall of the vacuum housing 2 and the uprising pipe 3 and downfalling pipe 4. For example, the uprising pipe 3 and the downfalling pipe 4 can each grow by over 2 in. when heated up to 1500.degree. C. Thus, there may be a gap where a substantial amount of air at atmospheric pressure can flow into the vacuum housing 2. Further, the insulating bricks 5 around the uprising pipe 3 and the downfalling pipe 4 expand as they absorb heat from the uprising pipe 3 and the downfalling pipe 4, but do so at a lower rate than the uprising pipe 3 and the downfalling pipe 4. Thus, some gaps may open between the insulating bricks 5, which can leave unsupported areas on the uprising pipe 3 and the downfalling pipe 4. The unsupported areas can rupture from the internal pressure the molten glass exerts in the pipes 3, 4.
Very few prior art references have addressed the problem of sealing between the expanding uprising and downfalling pipes and the wall of the vacuum housing and providing adequate support to prevent the pipes from rupturing from internal pressure. U.S. Pat. No. 5,851,258 issued to Ando et al. discloses a backup structure for uprising and downfalling pipes which convey molten material through a vacuum housing and metal bellows for sealing between the pipes and the vacuum housing. FIG. 2 illustrates the backup structure for a pipe 16, which could be the uprising or the downfalling pipe. The backup structure includes a supporting device 30 which is made up of a supporting plate 32 and a push-up means 36. Insulating bricks 28 are arranged around the uprising pipe 16 so as to surround the pipe. The leg portion 12A of the vacuum housing, which contains the pipe 16, is shaped in a rectangular prism-like cylindrical form, and supporting members 40 are fixed at the four corners of the leg portion 12A of the rectangular prism-like cylindrical form. The push-up means 36 connects the leg portion 12A to the supporting plate 32 and urges the supporting plate 32 upwardly to support the bricks 28. Annular flanges 16A are provided at predetermined intervals on the outer circumference of the pipe 16. The bricks 28 are each stacked between the flanges 16A. A recess 28A is formed at an upper surface of each of the bricks 28. The depth of each recess 28A is substantially the same as the thickness of the flange 16A, so that when the bricks 28 are stacked between the flanges 16A, each of the flanges 16A is received in the adjacent recess 28A.
Because the thermal expansion coefficient of the pipe 16, which is made of platinum, is higher than the thermal expansion coefficient of the bricks 28, the elongation of a section of the pipe 16 between adjacent flanges 16A is larger than that of the brick 28 arranged between the same adjacent flanges. Thus, the elongation in the axial direction of the section of the pipe 16 is restricted by the bricks 28, so that the section of the pipe 16 is deformed inwardly in a curved form. The elongation in the axial direction of the pipe 16, as a whole, corresponds to the elongation of the stacked bricks 28. The pipe 16 and bricks 28 expand downwardly against the urging force of the push-up means 36 when thermally expanded. There is a space S between the bricks 28 and the pipe 16 that can accommodate the thermal expansion of the pipe 16 in the circumferential direction. A cylindrical bellows 50 connects the leg portion 12A of the vacuum housing to the supporting plate 32, and the thermal insulation material, i.e., the bricks 28, is received in the cylindrical bellows 50 so as to keep the inside of the vacuum chamber in an airtight condition.